This invention relates to a control circuit for controlling the firing of a print hammer so as to maintain constant inter-character spacing when printing.
When specific styles of fonts such as E13B, OCR-A, CMC7, and OCR-B, for example, are printed on documents like checks and deposit slips used in the banking industry, for example, it is necessary that the printing be carried out with extreme accuracy in order to meet stringent requirements. These requirements are necessary to ensure that when the data printed is read subsequently by mechanical readers, it is read properly. A poorly or inaccurately printed character will not be read accurately.
The E13B font, for example, is a style of printing which is used by the banking industry in the U.S.A. and other countries for printing on checks and other financial documents. This font is printed with a magnetic ink, and the data printed thereby is generally referred to as MICR (magnetic ink character recognition) data.
A convenient way of printing MICR data in an E13B font is to utilize mechanical impact printing. The mechanical printing generally produces accurate, correctly-readable data. One of the ways of printing mechanically is to utilize a hammer which impacts against a typeface which has the character to be printed formed thereon.
Such mechanical printing is utilized, for example, in an encoder which prints MICR data on financial documents such as checks or deposit slips. Generally, a document to be printed upon is moved along a document track in the encoder to a print station therein where a transport drive roller is used to move the document at a constant velocity in the track during printing. The driver roller is rotated by a motor whose speed is monitored, for example, by a shaft encoder using a timing disc and associated circuitry. By using the rotational speed of the motor as a measure of the document's velocity at the print station, a hammer at the print station can be energized in synchronization with the document's velocity so as to provide constant spacing between the characters printed. This is generally true for printing at low speeds.
When the printing technique described in the previous paragraph is attempted at high speeds, irregular spacings between characters tend to occur. Because the document velocity at the print station is high when high printing speeds are attempted compared to the low speeds mentioned, any variation in document velocity at the high printing speeds will cause noticeable spacing irregularities. This is true because the hammer flight time, from energization to impact, for a particular hammer is constant, and at high printing speeds, the variation in velocity of a document at the print station has a greater effect on the associated total print cycle than it does at the slow printing speeds.